This invention relates to systems and methods for processing sheet material passing between operative rollers, and more particularly to rotary die systems for cutting repetitive shapes from sheet material.
A rotary or roller die cutting system is typically a part of what is called a rotary press structure, a number of versions of which are widely available. The roller die itself is a quite rigid body of cylindrical form having a cutting blade of a predetermined areal pattern that protrudes outwardly to a selected distance from the cylindrical circumference. The tip of the blade contacts or is separated by a small gap from an adjacent anvil or back-up member that also rotates. Sheet material, typically in the form of a substantially continuous strip, passing between the roller die and the anvil is thus intended to be cut into the pattern defined by the blade. The rotary press includes means for driving the roller die and anvil synchronously, an ostensibly stable restraint system which includes end bearing blocks, and means for feeding and tensioning the sheet material. The term "sheet material" as used herein is intended to encompass continuous and discontinuous stock, such as the different webs, strips and bands used in die cutting operations, whether they are in single or multiple layers, and also whether they are of paper, plastic or other materials.
When the sheet material is to be cut entirely through, this is called "zero tolerance" cutting and the blade tip must lightly contact or be very slightly spaced from the anvil. It is more commonly the case that only an adhesive backed surface layer on a laminate is to be cut, with the underlying substrate remaining uncut. This is referred to as "kiss cutting" and is the basis for making the adhesive backed peel-off labels and shapes which are very widely employed in the labeling and packaging industries.
The term "cutting" is not accurately descriptive, because the penetration of the cutting blade into the sheet introduces localized shear forces that locally strain and deform the material, while the blade is also compressing it against the anvil. These actions cause the sheet to separate along the line of the blade even though the blade edge has not penetrated fully through the given layer. The cut may be said to be made by "bursting" the uncontacted thickness of the sheet under the highly concentrated forces that are applied. Such shearing and bursting actions must be very precise, because the cutting effect on a particular sheet or laminate is dependent, to a first approximation, on the shape of the blade, its spacing relative to the anvil, and on the thickness, strength and elasticity of the material. There are also dynamic and static factors at play that affect the result, as described in more detail hereafter. Most of the sheet materials used are in the range of a few hundredths of an inch to a few ten thousandths of an inch in thickness. In practice it is usually found that the needed spacing (or "clearance") between the tip of the cutting blade and the surface of the anvil for proper cut or bursting must be maintained to within a few tenths of a thousandth (such as 0.0001 to 0.0005 inches). This precision must be maintained under actual operating conditions which involve wear, high reactive forces and dynamic changes.
In order to attempt to meet these requirements, roller die systems and rotary presses currently incorporate a number of features. The roller die and anvil are precisely formed, hardened cylindrical bodies, and the cutting blades are usually hard, precision finished at the tips, and, at least initially, of uniform height and blade profiles. The ends of the roller die and anvil are set in bearing blocks that provide some restraint. In modern practice, however, the roller die and anvil also include bearing surfaces, called "bearers", at or near the extremities of their cylindrical bodies. The bearers on the die are in contact with the anvil, and the anvil often is supported on the opposite side from the roller die by a back-up support roller. The designer selects the radial dimensions of the bearers on the roller die and anvil for a given application, to provide a chosen nominal blade-anvil clearance for the material that is to be cut. If the spacing conditions are not correct a different anvil body having a different diameter may be substituted. This changes the nominal clearance but usually does not overcome other problems, as discussed further below. Substituting a new roller die must be avoided if at all possible because of the expenses involved in die fabrication.
The principal mode of control of the clearance is by the use of preloading or compressive forces acting on the journals or bearers. The die cutting module in the press thus includes a pressure bar that spans the length of the roller die, parallel to its axis, and supports a force exerting mechanism, such as adjustable loading screws. This mechanism displaces pressure rollers down onto the roller die (typically the top surface of the bearers), compressing the die bearers against the opposing surfaces on the anvil. The forces exerted are ultimately absorbed by the bearing blocks, back-up support roller if any, and the relatively massive frame of the rotary press. Compression of the bearers displaces the blade tip of the roller die slightly but measurably, and reduces the clearance by a determinable amount.
Experience in the die cutting field has shown quite conclusively that the bearer feature is essential for satisfying the rigorous requirements imposed on modern die cutting systems. Installations without bearers have been shown to be largely unable to control depth of cut with the precision needed. Thus preservation of the bearer function is of paramount importance to improved systems for die cutting applications.
A further advantageous technique, introduced by the present applicant relative to the bearer system, involves the insertion of load cells in the loading screw-pressure roller system, to provide electrical signals via associated circuitry. The signals are used for analog or digital indications, and for actuating recording instruments. By these means the compressive forces can be equalized, and operating conditions can be monitored as they change.
Anvils and roller dies are typically of sizes such as six, twelve or sixteen inches in circumference. While one obtains greater stability with larger roller dies, the costs of the larger rotary elements place practical limits on use of this alternative. Moreover, the roller die cutting process is a dynamic one which involves many operative variables that are not at first apparent, and some variables which are so complex that they are not fully understood. There are times when materials cannot be cut satisfactorily without extensive trial and error, and this can be economically disastrous, particularly if dies and anvil have to be modified or substituted.
The dynamics of a die cutting process vary, for example, in accordance with the configuration of the cutting blade. The blade typically (but not necessarily) forms a closed loop pattern having vectorial components which vary at any angle between a direction parallel to the longitudinal axis of the roller die (the "cross cut" direction) and along a circumference of the roller die (the "machine" direction). Thus there will often be significant differences in the length of cutting blade that is in contact with the sheet material at any instant during a cycle. A long line contact in the cross cut direction introduces much higher reactive forces than the one or two contact points that exist when the blade segments are in the machine direction. Forces of reaction against the roller die and anvil can thus differ by many orders of magnitude primarily because of these variations in blade disposition. The forces exerted at any time are also dependent on a number of other factors, such as the sharpness and shape of the cutting blade, and the thickness and stiffness of the material. The reactive forces created can be so high in fact that in some instances they induce discernible bouncing of the roller die in synchronism with the rotation. Such factors reflect the great amount of "work" that must be exerted on the material. In practice it is found that the differences in reactive forces alone can cause incomplete kiss cutting in portions of a cutting pattern.
Wear on the cutting blade is dependent upon the nature of the sheet material and the amount of preloading used. As wear increases higher preloading must be used in order to assure continued cutting. While wear is to be expected, the use of high loading forces not only accentuates the wear, but tends to decrease roller die life, because the number of the times that the blade can be resharpened (a conventional procedure) is also reduced.
The effective clearance also changes appreciably during operation because of thermal expansion, as the friction and forces exerted heat up the roller die from a cold starting state. Considering the size of the typical roller die and anvil, and the very small effective clearance range that is permissible, it is clear that only a small amount of thermal expansion will induce excessively high pressures.
There are limits on the pre-loading forces that can be applied. Because there is a constant tendency during usage of a roller die to increase preloading with time, die life may be shortened by this limitation alone. If initial clearance is too great or too small, moreover, preloading adjustments are of no benefit, and this is another limitation on present systems.
It is evident, therefore, that a substantial need exists for systems and methods that enable control of the parts of a rotary die system so as to make the required minute clearance adjustments at the cutting region in a manner which enables a minimum but suitable preloading force to be exerted. Because roller die modules are usually only a small part of a pre-existing rotary press, which may include printing, drying and other stations, any new systems and methods should be fully compatible with existing rotary press systems both in physical and economic terms. Moreover, it should be possible to make the necessary adjustments dynamically, with the system in operation.